It is known in the art to provide a circuit for denying calling connections when a telephone station user dials digits representative of unauthorized station or area codes. A problem exists, however, when several stations are connectable to the same communication line and where it is desired that some of the stations have unrestricted calling capability. An example of such an arrangement is a key telephone system, such as the key telephone system disclosed in U.S. Pat. No. 3,436,488 issued to R. E. Barbato and D. T. Davis on Apr. 1, 1969.
In such a key telelphone system it is customary to have each line from the central office (CO) or PBX connectable to more than one key telephone station. In many such systems the telephone users at the different stations are given different dialing capabilities and, thus, merely detecting an improperly dialed code and denying the continuation of the call would result in the denying of calling connections to those who are permitted to place unrestricted calls.
A further problem results in situations where, as a result of a call being placed to a restricted area, the CO line polarity or PBX line polarity is reversed for the duration of the call. Under this situation, a call may be placed by a station having unrestricted call establishing authority and thereafter it may become desirable to have the calling connection maintained only from a station having restricted authority. Since the telephone line polarity has been reversed, any circuit which simply monitors such a reversal would operate to cut off the connection when the unrestricted station is no longer associated with the call. This is an undesirable situation.
Accordingly, a need exists in the art for a call denial circuit operable with multistation systems wherein those stations with unlimited calling authority may use the same CO line or PBX line as is used by a station having restricted dialing authority.
A further need exists in the art for such a call denial circuit wherein the assignment of restricted and nonrestricted capability to the associated stations can be easily and economically facilitated.
A further need exists in the art for a key telephone call denial circuit in which all calling connections, once established, can be maintained by restricted as well as unrestricted stations.